Iguana iguana invasion in Puerto Rico:
facing the evidence
Javier A. Arce-Nazario & Tomás
A. Carlo
Biological Invasions
ISSN 1387-3547
Biol Invasions
DOI 10.1007/s10530-012-0196-y
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Biol Invasions
DOI 10.1007/s10530-012-0196-y
LETTER TO THE EDITOR
Iguana iguana invasion in Puerto Rico: facing the evidence
Javier A. Arce-Nazario • Tomás A. Carlo
Received: 17 October 2011 / Accepted: 12 February 2012
Ó Springer Science+Business Media B.V. 2012
The recent article by López-Torres et al. (2012)
published in Biological Invasions presents valuable
natural history data on the reproductive biology,
phenology, and demography of the Green Iguana
(Iguana iguana) in Puerto Rico, but a misleading
profile of its population densities, as well as an
exaggeration of the potentially negative ecological
impacts that this species could have in Puerto Rican
ecosystems. There are two main points that we wish to
clarify. First, we underscore that the densities reported
in the study are incorrect because of the choice of
census method, and we provide evidence that I. iguana
densities in Puerto Rico do not surpass ‘‘normal’’
values in other locations within its native range.
Second, we note that portraying I. iguana as a
dangerous predator for native fauna is an exaggeration. Before concluding, we also review prior results
regarding other claims in the article that may feed
popular myths and misconceptions about this species
in Puerto Rico and elsewhere.
J. A. Arce-Nazario (&)
Biology Department and Institute of Interdisciplinary
Research, University of Puerto Rico at Cayey,
205 Ave. Antonio R. Barceló, Cayey,
PR 00736-9997, USA
e-mail: javier.arce@upr.edu
T. A. Carlo
208 Mueller Lab, Penn State University,
University Park, PA 16802, USA
e-mail: tac17@psu.edu
Density measurements
A central argument by López-Torres et al. (2012) in
their call for management of I. iguana in Puerto Rico is
that ‘‘densities of I. iguana in Puerto Rico reached a
maximum of 223 individuals ha-1, higher than in any
known locality in its native range.’’ In fact, this density
is less than the 364 individuals ha-1 found in Rodda
(1992), in a Venezuelan region where I. iguana is
native. This figure is well-known in the literature and it
appears in two of the citations in López-Torres et al.
(2012): in Table 1 of the original article (Rodda 1992)
and in the discussion section of Muñoz et al. (2003).
It is important to note that comparing values obtained
by the two studies (Rodda 1992; López-Torres et al.
2012) is possible because in both studies iguanas were
tallied along vegetation edges next to water bodies. In
the case that the densities reported by Rodda (1992) are
omitted by López-Torres et al. (2012) not because the
numbers were overlooked but because the authors
disagree with the methods used by Rodda (1992), then
the basis for dismissing this important value—if any—
should be discussed. Regardless of whether this omission was an error or a deliberate action, it is also obvious
that the choice of plot geometry inflates the density
estimates in the study. For example, if sampling plot
geometries that are less sensitive to edges were used—
say 100 9 100 m plots instead of 10 9 1,000 m
plots—the density values would likely be much lower
since green iguanas are well known to congregate at the
edge of mangroves, riparian forests, and shoreline
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J. A. Arce-Nazario, T. A. Carlo
habitats (Hallinan 1920; Henderson 1976; Muñoz et al.
2003; Rand et al. 1989; Rodda 1992). Moreover, the
Canal Blasina and Parque Lineal are two of the three
sites known to have the highest green iguana densities in
the entire San Juan Bay Estuary (Carlo and Garcı́aQuijano 2008; Garcı́a-Quijano et al. 2011), thus the
reported values are not representative of the mean values
for the habitat.
Green iguanas as predators
It must be emphasized that I. iguana are herbivorous at
all life stages, and the incidents of iguanas eating
animals are rare. In contrast, López-Torres et al.
(2012) portray the iguana as a possible predator that
could threaten native fauna. The authors of the article
write in their introduction that ‘‘I. iguana adults are
mostly herbivorous, but occasional predation on bird
eggs, chicks, invertebrates, small mammals and carrion have been reported.’’ I. iguana are herbivorous
when juveniles and when adults. An article from
Swanson in 1950, and Hirth in 1963 suggested that
I. iguana could be omnivorous when juvenile. Subsequent long-term research showed that I. iguana are
herbivorous at all stages (Troyer 1984). The herbivory
of I. iguana is demonstrated by eight studies of
stomach content of wild animals (Gómez-Carrasquillo
et al. 2006; Hirth 1963; Lara-López and GonzálezRomero 2002; McBee and McBee 1982; Rand et al.
1990; Swanson 1950; Troyer 1984; Van Devender
1982). The only published study on I. iguana stomach
contents that showed animal matter is a study by
Townsend et al. (2005) in Florida, in which two of the
18 stomachs studied contained snails that live attached
to foliage. Currently there are only two peer-reviewed
articles that report I. iguana predating animals: bird
eggs (Lazell 1973) and insects (Hirth 1963). There are
two additional articles with direct field observations of
I. iguana scavenging: rotten egret eggs (Arendt 1986)
and opossum carrion (Loftin and Tyson 1965). Other
articles that present data on animal consumption cite
personal communications or a reference to one of the
aforementioned articles. However, these observations
need to be leveled against the more than ten studies on
I. iguana physiology, diet, and behavior in different
regions, which have only recorded iguanas eating
vegetable material, and thus point to an extreme rarity
of animal items in their diets (e.g. Garcı́a-Quijano
123
et al. 2011; Hallinan 1920; Henderson 1974;
Lara-López and González-Romero 2002; Marken
Lichtenbelt 1993; Morales-Mávil et al. 2007; Rand
1968; Rodda 1992; Swanson 1950; Troyer 1984).
The authors underscore the potential predatory
behavior of green iguanas close to the end of the
article, indicating:
Iguana iguana have been reported to feed on
bird eggs and chicks (Schwartz and Henderson
1991; Rivero 1998), and we have been informed
of adults feeding on common moorhen (Gallinula
chloropus) eggs at Parque Lineal (René Fuentes,
personal communication).
We argue that statements of this kind only promote
myths about I. iguana, and an incorrect perception of
their most likely effects in communities (Garcı́aQuijano et al. 2011). Research has shown zero iguana
predation of artificial nests with fresh eggs placed in
mangroves with high I. iguana densities (i.e., Caño
Martı́n Peña) that were monitored with hunting
cameras activated by motion and/or heat in day and
night. The 1,896 h of video showed egrets, iguanas
and rats visiting the nests, with only rats being
associated with egg predation (Carlo and Garcı́aQuijano 2008). This study was cited in López-Torres
et al. (2012), but the evidence against the importance
of I. iguana as a predator was not noted.
Other claims
There is a long history of research on I. iguana that can
serve the scientific community in objectively assessing
their possible impact on new environments. LópezTorres et al. (2012) includes other claims to emphasize
the threat posed by I. Iguana, many of which overlook
the results of this prior work. First, the article indicates
that there is no natural predator of I. iguana in Puerto
Rico. In fact, Puerto Rican ecosystems do include
native animals that have been identified as natural
predators of I. iguana. Bird species such as Pandion
haliaetus, Falco sparverius and Nyctanassa violacea,
and hawks of the genus Buteo have been all documented as natural predators of I. iguana (Greene et al.
1978; Rivas et al. 1998; Engeman et al. 2005). In
addition, species of the genera Alsophis and Epicrates
have been reported as predators of Caribbean iguanids
including Cyclura and Iguana (Grant 1932; Henderson
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Iguana iguana invasion
and Powell 2009). Another potential threat raised by
López-Torres et al. (2012) is that the I. iguana could be
destroying the nests of the critically endangered
leatherback turtles (Dermochelys coriacea). In evaluating this threat, it is important to consider that Puerto
Rico is not the only place where green iguanas and
leatherback turtles coexist. I. iguana is considered
native to many coastal regions in Central America and
South America as well as other Caribbean islands with
leatherback turtle nesting sites. There has been no
previous claim, to our knowledge, of green iguanas
destroying the nests of leatherback turtles or affecting
turtle populations, while in Panamá I. iguana have
been observed to remove the eggs of crocodile
(Crocodylus acutus) nests (Rand 1968). The only
conservation threat posed by I. iguana to another
Caribbean species for which we have found documented evidence is the possible effect of hybridization
of I. iguana with Iguana delicatissima (Breuil 2002). In
stressing other dangers which are either unsubstantiated or contradicted by prior work, the article by
López-Torres et al. (2012) reflects and reinforces an
unfortunate trend in popular media (newspaper articles, blogs, and even scientific articles) to exaggerate
the negative impact of green iguanas on the ecosystem.
In conclusion, the article by López-Torres et al.
(2012) contains valuable natural history information
about I. iguana in Puerto Rico. Nevertheless we believe
that the authors should temper their conclusions, given
the non-random selection of study sites, and the
selective presentation and interpretation of published
data. We agree with the authors that management of
I. iguana in Puerto Rico is an important conservation
issue, particularly because they can become garden and
agricultural pests. However, so far there is very little
empirical basis for fearing that I. iguana is a serious
threat to native species of flora and fauna in Puerto Rico.
Acknowledgments The authors would like to thank the
librarians of the University of Puerto Rico, especially Nı́vea
Santiago Vázquez and Elizabeth Borges Ocasio for their excellent
help in locating references. Thanks also to the Biology
Department of Penn State University for support to TA Carlo.
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